► Thermally activated shape memory polymers are a desirable material for use in dynamic structures due to their large strain recovery, light weight, and tunable activation.…
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▼ Thermally activated shape memory polymers are a desirable material for use in dynamic structures due to their large strain recovery, light weight, and tunable activation. The addition of ferromagnetic susceptor particles to a polymer matrix provides the ability to heat volumetrically and remotely via induction. Here, remote induction heating of magnetite filler particles dispersed in a thermoset matrix is used to activate shape memory polymer as both solid and foam composites. Bulk material properties and performance are characterized and compared over a range of filler parameters, induction parameters, and packaging configurations. Magnetite filler particles are investigated over a range of power input, in order to understand the effects of particle size and shape on heat generation and flux into the matrix. This investigation successfully activates shape memory polymers in 10 to 20 seconds, with no significant impact of filler particles up to 10wt% on mechanical properties of shape memory foam. Performance of different particle materials is dependent upon the amplitude of the driving magnetic field. There is a general improvement in heating performance for increased content of filler particles. Characterization indicates that heat transfer between the filler nanoparticles and the foam is the primary constraint in improved heating performance. The use of smaller, acicular particles as one way to improve heat transfer, by increasing interfacial area between filler and matrix, is further examined.
Advisors/Committee Members: Garmestani, Hamid (Committee Chair), Gall, Ken (Committee Member), Thadhani, Naresh (Committee Member).

► The successful use of Nickel-Titanium (Nitinol) in biomedical applications requires an accurate control of its unique mechanical properties. The purpose of this study is to…
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▼ The successful use of Nickel-Titanium (Nitinol) in biomedical applications requires an accurate control of its unique mechanical properties. The purpose of this study is to analyze the effects of a wide range of heat treatments on the mechanical behavior of hot-rolled and cold-drawn Nitinol. Results comprise an understanding of the effect of heat treatment temperature and time variation on final material response which is imperative for optimization of material properties. Thirty-three heat treatment variations are tested by combining three durations, 10 minutes, 90 minutes, and 8 hours, with eleven different heat treatment temperatures between 200°C and 440°C. Following heat treatment, the Nitinol samples undergo tensile testing with upper plateau strength, lower plateau strength, ultimate tensile strength, strain to failure, and residual elongation compared for all test groups.
Heat treatment "power" is used to describe the efficacy of different combinations of heat treatment temperature and duration. When using hot-rolled Nitinol, results show a low heat treatment power does not create significant precipitation hardening or a significant decrease in martensite transformation stress, resulting in a high upper plateau strength, high residual strain values, and evidence of plastic deformation upon unloading. Moderate power treatments lead to sufficient hardening of the material and a decrease in martensite transformation stress resulting in a pseudoelastic response. Increasing to a high treatment power further decreases the transformation stress and increases the martensite transformation temperature leading to a shape-memory response in hot rolled Nitinol. When using cold-drawn Nitinol, low and moderate heat treatment power levels result in the material exhibiting a pseudoelastic response. Increasing heat treatment power shows the same effects on martensite transformation stress and temperature as seen with the hot-rolled material resulting in a material response transition from pseudoelastic to shape memory.
Advisors/Committee Members: Gall, Kenneth (Committee Chair), McDowell, David (Committee Member), Thadhani, Naresh (Committee Member).

► With an increasing demand for better fuel efficiency and better crashworthiness, automotive and steel industries in the recent past have seen enormous interest in developing…
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▼ With an increasing demand for better fuel efficiency and better crashworthiness, automotive and steel industries in the recent past have seen enormous interest in developing thinner and stronger Advanced High Strength Sheet (AHSS) steels. During forming processes, AHSS steels can undergo deformation at 10-100/sec strain rate, while in a crash they can experience deformation at around 1000/sec. Therefore, it is important to understand their deformation at slow as well as high strain rates while designing automotive body parts. Most studies in this area have been in understanding the mechanical response of AHSS under different loading conditions. Some studies have also been done to qualitatively study the fracture surfaces, to understand the mechanisms of failure in these steels. However, very few quantitative studies have been done to understand these mechanisms of failure. Therefore, the objective of this study was to quantitatively understand the effects of microstructure and strain rate on the deformation behavior of three different grades of AHSS steels; HSLA 590, Ductibor® 500 and Usibor® 1500. Differences in microstructures were achieved using different grades of steel, as well as by changing the austenitization conditions of hot-stamping process. Microstructures were quantified for surface area per unit volume of different interfaces, volume fraction of different microstructural constituents, and length per unit volume of grain edges (for HSLA 590). HSLA 590 specimens were tested at various strain rates ranging from 10-4/sec to 3200/sec and their mechanical response were studied. Similarly, mechanical response of Ductibor 500 and Usibor 1500 were studied for strain rates ranging from 10-4/sec to 1000/sec. Various micro-mechanisms of failure were quantified using quantitative fractography and digital image analysis. Mechanical testing data shows that tensile properties of HSLA 590 are most strain rate sensitive. Tensile properties of Usibor 1500 are least sensitive to changing strain rates. Tensile properties of Ductibor 500 are more sensitive to hot-forming process parameters compared to Usibor 1500. Also, some fracture micro-mechanisms have been seen to vary significantly with processing conditions, even though their effects on mechanical behavior is minimal.
Advisors/Committee Members: Gokhale, Arun M. (advisor), Bhat, Shrikant P. (committee member), Thadhani, Naresh M. (committee member).

► The effects of variations in composition on the decomposition process in Al-Zn-Mg-Cu alloys (i.e. – 7xxx-series aluminum alloy) were studied emphasizing their effect on mechanical…
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▼ The effects of variations in composition on the decomposition process in Al-Zn-Mg-Cu alloys (i.e. – 7xxx-series aluminum alloy) were studied emphasizing their effect on mechanical properties. Several experimental quaternary alloys were studied to compare their behavior with commercial 7xxx-series alloys. The investigation included studies on the effects of natural aging, artificial aging, quench sensitivity, precipitate free zone formation, and homogenization. Additionally, “true aging” curves (i.e. – hardness/strength vs. conductivity) were presented in order to visualize and quantify the entire precipitation process.
It is obvious that fluctuations in the main alloying elements/processing parameters can alter the precipitation process, but the purpose of this work was to quantify those changes using standard industrial techniques. It was found that natural aging was detrimental for strength in the T6 temper for alloys containing more than 1.0 wt.% Cu, and was shown to alter the coarsening kinetics in the over-aged condition (T7). Conversely, for alloys with Cu contents less than 0.5% natural aging was shown to be beneficial for strength. Altering the Zn:Mg ratio was also shown to effect natural aging response of an alloy in addition to introducing additional precipitation processes (T-phase). Therefore, this work is a blueprint for advanced alloy manufacturing that allows for the rapid production of new alloys and tempers by narrowing the research focus depending on an alloy’s composition.
Advisors/Committee Members: Sanders, Thomas H. (advisor), Thadhani, Naresh (committee member), Gokhale, Arum (committee member), Singh, Preet (committee member), Dangerfield, Vic (committee member).

▼ We demonstrate intracellular delivery of various molecules by inducing controlled and reversible cell damage through pulsed laser irradiation of carbon black (CB) nanoparticles. We then characterized and optimized the system for maximal uptake and minimal loss of viability. At our optimal condition 88% of cells exhibited uptake with almost no loss of viability. In other more intense cases it was shown that cell death could be prevented through addition of poloxamer.
The underlying mechanism of action is also studied and our hypothesis is that the laser heats the CB leading to thermal expansion, vapor formation and/or chemical reaction leading to generation of acoustic waves and then there is energy transduction to the cell causing poration of the cell membrane.
We also delivered anti-EGFR siRNA to ovarian cancer cells. Cells exposed to a laser at 18.75 mJ/cm2 for 7 minutes resulted in a 49% knockdown of EGFR compared to negative control. We established an alternative way to deliver siRNA to knockdown proteins, for the first time using laser CB interaction.
Advisors/Committee Members: Prausnitz, Mark R. (advisor), Thadhani, Naresh N. (committee member), Champion, Julie A. (committee member), Styczynski, Mark P. (committee member), Sambanis, Athanassios (committee member).

► Investigations of the production of thin-walled steel alloys through the reduction and subsequent gas carburization of structures made from metal oxide powders were performed. Batch…
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▼ Investigations of the production of thin-walled steel alloys through the reduction and subsequent gas carburization of structures made from metal oxide powders were performed. Batch compositions, as well as the heat treatment parameters necessary for the formation of structures were determined through the use of thermogravimetric analysis, dilatometric measurements, and microstructural investigation. Parameters for the high temperature carburization of thin-walled 4140 structures were determined. The research has shown that the amount of carbon in the walls of the structures can be controlled and uniform carbon contents across the cross-sections can be achieved in less than 30 minutes. Heat treatments for carburized samples were performed and subsequent microhardness testing resulted in values similar to conventionally produced 4140 steel. Studies on the decarburization behavior of similar alloys under various conditions were also performed in order to aid in the prediction of the microstructural behavior of samples during carburization and subsequent heat treatment. Low temperature gas carburization of structures with 316 steel composition has also been performed. Hardness variations present through the cross-section of the part after carburization suggest some transfer of carbon, though contents are not as high as anticipated. Suggestions for future work in this area are presented. The results of these investigations yield a novel method for the production of steel parts from metal oxide powders. The speed and low cost of the process, coupled with the proven ability of the process to yield parts with similar microstructural and mechanical characteristics as conventionally made alloys, allows for the techniques presented in this study to be used for the development of alloys which could not be previously done economically.
Advisors/Committee Members: Sanders, Thomas (Committee Chair), Cochran, Joe (Committee Member), McDowell, David (Committee Member), Singh, Preet (Committee Member), Thadhani, Naresh (Committee Member).

► The focus of this work is on the modeling and simulation of shock wave propagation in reactive metal powder mixtures. Reactive metal systems are non-explosive,…
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▼ The focus of this work is on the modeling and simulation of shock wave propagation in reactive metal powder mixtures. Reactive metal systems are non-explosive, solid-state materials that release chemical energy when subjected to sufficiently strong stimuli. Shock loading experiments have demonstrated that ultra-fast chemical reactions can be achieved in certain micron-sized metal powder mixtures. However, the mechanisms of rapid mixing that drive these chemical reactions are currently unclear. The goal of this research is to gain an understanding of the shock-induced deformation that enables these ultra-fast reactions. The problem is approached using direct numerical simulation. In this work, a finite element (FE) model is developed to simulate shock wave propagation in discrete particle mixtures. This provides explicit particle-level resolution of the thermal and mechanical fields that develop in the shock wave. The Ni/Al powder system has been selected for study. To facilitate mesoscale FE simulation, a new dislocation-based constitutive model has been developed to address the viscoplastic deformation of fcc metals at very high strain rates. Six distinct initial configurations of the Ni/Al powder system have been simulated to quantify the effects of powder configuration (e.g., particle size, phase morphology, and constituent volume fractions) on deformation in the shock wave. Results relevant to the degree of shock-induced mixing in the Ni/Al powders are presented, including specific analysis of the thermodynamic state and microstructure of the Ni/Al interfaces that develop during wave propagation. Finally, it is shown that velocity fluctuations at the Ni/Al interfaces (which arise due to material heterogeneity) may serve to fragment the particles down to the nanoscale, and thus provide an explanation of ultra-fast chemical reactions in these material systems.
Advisors/Committee Members: McDowell, David (Committee Chair), Horie, Yasuyuki (Committee Member), Qu, Jianmin (Committee Member), Thadhani, Naresh (Committee Member), Zhou, Min (Committee Member).

▼ This dissertation focuses on developing a predictive method for determining the dynamic
densification behavior of thermite powder mixtures consisting of equivolumetric
mixtures of Ta + Fe₂O₃ and Ta + Bi₂O₃. Of primary importance to these highly reactive
powder mixtures is the ability to characterize the stress at which full compaction occurs,
the crush strength, which can significantly influence the stress required to initiate reaction
during dynamic or impact loading. Examined specifically are the quasi-static and dynamic
compaction responses of these mixtures. Experimentally obtained compaction responses
in the quasi-static regime are analyzed using available compaction models, and an analysis
technique is developed that allows for a correct measurement of the apparent yield strength
of the powder mixtures. The correctly determined apparent yield strength is combined with
an equation of state to yield a prediction of the shock densification response, including the
dynamic crush strength of the thermite powder mixtures. The validated approach is also extended
to the Al + Fe₂O₃ thermite system. It is found that accurate predictions of the crush
strength can be obtained through determination of the apparent yield strength of the powder
mixture when incorporated into the equation of state. It is observed that the predictive
ability in the incomplete compaction region is configurationally dependent for highly heterogeneous
thermite powder systems, which is in turn influenced by particle morphology
and differences in intrinsic properties of constituents (density, strength, etc.).
Advisors/Committee Members: Thadhani, Naresh (Committee Chair), Cochran, Joe (Committee Member), Sanders, Tom (Committee Member), Vogler, Tracy (Committee Member), Zhou, Min (Committee Member).

► Remora fishes are capable of rapid, reversible, and robust attachment to a wide variety of marine hosts both natural and artificial with widely varying geometric…
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▼ Remora fishes are capable of rapid, reversible, and robust attachment to a wide variety of marine hosts both natural and artificial with widely varying geometric and material properties. Despite its unique abilities, the mechanisms responsible for remora attachment have received little attention in scientific literature in comparison to the number of works commenting on it. The objective of this work is to identify and quantify the behavior and limitations of the critical mechanisms responsible for remora attachment. Traditional dissection techniques were combined with high-resolution three-dimensional scans to characterize and identify critical structural metrics pertaining to remora morphology. The structural metrics were incorporated into simulations to predict remora behavior during attachment. Finally, experimental methods were performed on artificial tissues to validate model predictions when necessary. The work is of value to both the engineering and biological communities through the creation of design tools, analyses, data sets, and simulations that provide both quantitative design data for bioinspired devices and/or methodologies, but also insight into the behavior of the remora itself.
Advisors/Committee Members: Thadhani, Naresh (advisor), Nadler, Jason H. (committee member), Flammang, Brooke E. (committee member), Hu, David (committee member), Alexeev, Alexander (committee member).

► The research performed in this work was aimed at investigating pressure-induced phase changes in a Ce-based metallic glass (MG) through the use of laser-driven shock…
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▼ The research performed in this work was aimed at investigating pressure-induced phase changes in a Ce-based metallic glass (MG) through the use of laser-driven shock experiments and atomic resolution structural characterization. MGs exhibit very high strength, have intrinsically low density, and plastically deform by shear banding. MGs are also metastable and can undergo phase changes upon heating and/or application of high pressure into higher density configurations. The atomic structure changes concomitant with these phase transitions occurring during high pressure shock compression are not well understood, which provides the motivation for the present work. Thermal analysis of Ce3Al MG melt-spun ribbons was first performed to characterize the crystallization response and structure. Ce3Al MG was found to strongly resist growth of crystallites but easily nucleate. Thermal crystallization occurs via a two-stage primary path wherein a metastable phase forms and converts fully into the hexagonal-intermetallic α-Ce3Al. The Avrami number and dimensionality constants indicate the crystallization occurs via plate-like growth, resulting in thermally crystallized grains on the order of 6 nm and a density ~4% greater than the reference α-Ce3Al. Shock compression experiments performed using the Nd:YAG 3 J laser and velocity interferometry allowed for in operando measurements of particle velocity coupled with sample recovery for structural analysis. The results provide a clear indication of the Hugoniot Elastic Limit (at ~1.8 GPa) as evidenced by the presence of a two wave structure in the velocity profile. At shock pressures exceeding the elastic limit, plastic deformation of the Ce3Al MG occurs via structural transformation to the crystalline state forming α-Ce3Al with nanocrystalline grain sizes, higher densities, and plate-like growth. The trends suggest that shock compression causes break-up of grains, higher densities due to Ce 4f delocalization, and increased preferred orientation. Shock compression experiments were also performed using the 50 J Omega laser facility at the Laboratory for Laser Energetics. A stack of samples was shock-compressed with pressures progressively decreasing across the stack thickness, resulting in two regimes of recovered samples. Highly deformed and partly damaged samples close to the shock front showed varying degrees of long-range order, medium-range order, and short-range order with distance away from the shock front. Visually undeformed samples showed decreased bond lengths for the nearest-neighbors, second nearest-neighbors, and fourth nearest-neighbors but increased bond lengths for the third nearest-neighbors, with associated densification of ~2-6% in all layers. These changes in the undeformed samples are indicative of polyamorphism. The visually undeformed samples also reveal an increase in magnitude of structural change with increased distance away from the shock-front, up to a maximum beyond which increasing distance decreases the magnitude of the bond length shifts. This trend is…
Advisors/Committee Members: Thadhani, Naresh N. (advisor), Alamgir, Faisal (committee member), Kacher, Josh (committee member), Gokhale, Arun M. (committee member), Wehrenberg, Christopher (committee member).

► Heterogeneous materials play important roles in many different applications across a wide range of industries. Examples include engineered composites, particulate systems, and energetic materials, which…
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▼ Heterogeneous materials play important roles in many different applications across a wide range of industries. Examples include engineered composites, particulate systems, and energetic materials, which all display complex meso-scale features and behaviors. This complexity leads to significant gaps in the understanding of heterogeneous materials, especially under extreme conditions such as shock-compression. A fundamental challenge in this area of research is a lack of experimental diagnostics that can provide spatially-resolved information under the demanding temporal and environmental conditions of shock loading. Multilayer optical structures, due to their unique spectral responses that can be correlated to externally induced loads, have the potential to serve as a new class of sensor for these complex materials and conditions.
This work presents the theory, development, and evaluation of novel multilayer optical structures as time-resolved pressure sensors with meso-scale spatial sensitivity. Time-resolved spectroscopy of laser-driven shock-compression experiments on the multilayers demonstrated spectral shifts of the characteristic spectral peaks to shorter wavelengths (blueshifts), and simultaneous velocimetry established that these spectral shifts are unambiguously correlated to the laser-driven shock pressure. An optomechanical model was developed and used to predict the spectral response of the multilayers as a function of pressure, and when informed with quality empirical data, quantitatively matches the experimentally observed blueshift. Experiments and simulations of spatially heterogeneous shock loading demonstrate the ability of the multilayers to resolve not only multiple pressures but also to capture the subtle features present in shock-compressed heterogeneous materials, all while maintaining nano-second level temporal resolution. Overall, multilayer-based sensing is a fundamentally new time-resolved diagnostic method in the fields of high-strain-rate material behavior and shock physics. This work has provided the theoretical and empirical foundation for broad classes of different multilayer structures, and demonstrated their unique potential utility for capturing the complex meso-scale pressure histories needed to enable new insights into the dynamic response of heterogeneous materials.
Advisors/Committee Members: Thadhani, Naresh (advisor), Summers, Christopher J (committee member), Springer, Keo (committee member), Kang, Zhitao (committee member), Jang, Seung Soon (committee member).

► A laser-driven miniflyer system is built in design similar to those at the Los Alamos National Laboratory and Eglin Air Force Base. It is composed…
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▼ A laser-driven miniflyer system is built in design similar to those at the Los Alamos National Laboratory and Eglin Air Force Base. It is composed of three parts: laser drive source, impact experiment assembly, and diagnostics. The laser drive source is a Nd:YAG laser operating at 1064nm at a maximum energy of 3 J. The impact experiment assembly consists of a BK7 substrate on to which is deposited an ablation layer consisting of carbon, alumina, and aluminum. Mounted on the ablation layer is a metal foil (flyer). The carbon in the ablation layer absorbs the laser energy to form a rapidly expanding plasma. The alumina and aluminum layers provide thermal insulation and also contain the plasma. The set-up is expected to provide flyer velocities in the range of 100 to 1000 m/s. Diagnostics consist of a Photonic Doppler Velocimetry (PDV) system that uses Doppler-shifted coherent laser light to measure the instantaneous velocity of a moving surface, as well as velocity dispersions caused by mechanical or material heterogeneities. This thesis will provide a description of the set-up of the laser-driven miniflyer system, as well as an evaluation of the flyer velocity, measured using the PDV system, as a function of laser energy. The flyer velocity trends will be used in order to characterize and calibrate the system. A manual providing system operation instructions will also be included to serve future users of this miniflyer system
Advisors/Committee Members: Thadhani, Naresh (Committee Chair), Das, Suman (Committee Member), Fajardo, Mario (Committee Member), Zhou, Min (Committee Member).

► The objective of this work is to evaluate the reaction initiation characteristics of quasi-statically compressed intermetallic-forming aluminum-based reactive materials upon impact initiation, consisting of equi-volumetric…
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▼ The objective of this work is to evaluate the reaction initiation characteristics of quasi-statically compressed intermetallic-forming aluminum-based reactive materials upon impact initiation, consisting of equi-volumetric tantalum-aluminum, tungsten-aluminum, nickel-aluminum, and pure aluminum. A modified Taylor rod-on-anvil setup was employed to determine the reaction initiation threshold kinetic energy and actual energy for plastic deformation and subsequent reaction. Experimental sample remnants were recovered and examined through X-ray diffraction to determine reaction products.The overall results indicate that of the various intermetallic-forming systems investigated, Ta+Al was the most reactive and was the only system where any reaction products were retrieved. While all of the intermetallic systems reacted in air, only Ta+Al and W+Al reacted in vacuum environment suggesting differences in reaction mechanisms influencing the reactivity of intermetallic mixtures. Based on the threshold energy for onset of reaction it appears that the Ta-Al compacts show reaction conditions below those required for reaction of Al in air. This combined with the fact that Ta+Al compacts also react in vacuum implies that the Ta+Al undergoes anaerobic intermetallic reaction while the other systems react with the oxidation of Al. The effect of compact packing density on the kinetic energy threshold for reaction initiation were also evaluated. It was observed more densely packed Ta+Al and Ni+Al powder compacts react more easily than less densely packed samples. While the effect of packing density is not as obvious in the case of pure Al and W+Al powder compacts. Finally, a particle size effect is seen on Ni+Al on samples of < 92% density where coarser (+325 -200 mesh) equal-volumetric powder mixtures were observed to be more reactive than finer Ni+Al (-325 mesh).
Advisors/Committee Members: Thadhani, Naresh (Committee Chair), Antoniou, Antonia (Committee Member), Cochran, Joe (Committee Member), Gall, Ken (Committee Member).

► The overall purpose of this dissertation is to develop a multi-scale framework that can simulate radiation defect accumulation across a broad range of time and…
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▼ The overall purpose of this dissertation is to develop a multi-scale framework that can simulate radiation defect accumulation across a broad range of time and length scales in metals.
In order to accurately describe defect accumulation in heterogeneous microstructures and under complex irradiation conditions, simulation methods are needed that can explicitly account for the effect of non-homogeneous microstructures on damage accumulation. In this dissertation, an advanced simulation tool called spatially resolved stochastic cluster dynamics (SRSCD) is developed for this purpose. The proposed approach relies on solving spatially resolved coupled rate equations of standard cluster dynamics methods in a kinetic Monte Carlo scheme. Large-scale simulations of radiation damage in polycrystalline materials are enabled through several improvements made to this method, including a pseudo-adaptive meshing scheme for cascade implantation and implementation of this method in a synchronous parallel kinetic Monte Carlo framework. The performance of the SRSCD framework developed in this dissertation is assessed by comparison to other simulation methods such as cluster dynamics and object kinetic Monte Carlo and experimental results including helium desorption from thin films and defect accumulation in neutron-irradiated bulk iron. The computational scaling of the parallel framework is also investigated for several test cases of irradiation conditions. SRSCD is next used to investigate radiation damage in three main types of microstructures, using α-iron as a test material: iron thin films, coarse-grained bulk iron, and nanocrystalline iron. SRSCD is used to investigate the mechanisms involved with defect accumulation in irradiated materials, such as effective diffusivity of helium in thin films and the effect of grain boundary sink strength on defect accumulation in nano-grained metals, and to predict defect populations in irradiated materials for comparison with experiments. Particular emphasis is placed on the role of microstructural features such as free surfaces and grain boundaries in influencing damage accumulation. Finally, the methodology developed in this dissertation is applied in the context of multiscale modeling and experimental design. To complete the multi-scale transition between defect-level behavior and macroscopic material property changes caused by irradiation, the relationship between mechanical loading and radiation damage is investigated. The impact of radiation damage on hardening of irradiated materials is investigated by using the results of SRSCD as inputs into polycrystalline crystal plasticity simulations. This is carried out in bulk iron by fitting hardening models to experimental data from neutron irradiation of iron and then used to predict hardening under irradiation conditions beyond what has already been accomplished in experimental studies. In addition, SRSCD is used to demonstrate the temperature shift required to achieve equivalent damage accumulation in irradiation conditions with…
Advisors/Committee Members: Capolungo, Laurent (advisor), McDowell, David L. (committee member), Thadhani, Naresh (committee member), Deo, Chaitanya (committee member), Dingreville, Rémi (committee member), Martínez-Saez, Enrique (committee member).

► A constitutive model has been developed to model the shock response of single crystal aluminum from peak pressures ranging from 2-110 GPa. This model couples…
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▼ A constitutive model has been developed to model the shock response of single crystal aluminum from peak pressures ranging from 2-110 GPa. This model couples a description of higher-order thermoelasticity with a dislocation-based viscoplastic formulation, both of which are formulated for single crystals. The constitutive model has been implemented using two numerical methods: a plane wave method that tracks the propagating wave front; and an extended one-dimensional, finite-difference method that can be used to model spatio-temporal evolution of wave propagation in anisotropic materials. The constitutive model, as well as these numerical methods, are used to simulate shock wave propagation in single crystals, polycrystals, and pre-textured polycrystals. Model predictions are compared with extensive existing experimental data and are then used to quantify the influence of the initial material state on the subsequent shock response. A coarse-grained model is then proposed to capture orientation-dependent deformation heterogeneity, and is shown to replicate salient features predicted by direct finite-difference simulation of polycrystals in the weak shock regime. The work in this thesis establishes a general framework that can be used to quantify the influence of initial material state on subsequent shock behavior not only for aluminum single crystals, but for other face-centered cubic and lower symmetry crystalline metals as well.
Advisors/Committee Members: McDowell, David L. (advisor), Clayton, John D. (committee member), Zhou, Min (committee member), Rimoli, Julian J. (committee member), Thadhani, Naresh N. (committee member), Xia, Shuman (committee member).

► There is a need for simulation methodologies for multiphase three-dimensional microstructures that can be used in numerical simulations of material behavior or in exact computation…
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▼ There is a need for simulation methodologies for multiphase three-dimensional microstructures that can be used in numerical simulations of material behavior or in exact computation of effective properties using microstructural correlation functions. Specifically, the methodology must be able to generate verifiably realistic microstructures, with complex morphology accurately represented.
Striving to address that need, the research presented here develops a general microstructure simulation toolbox for multiphase two- and three-dimensional microstructures consisting of one connected phase and one or more particulate phases. Previous work by other researchers has found successful solutions to a variety of special cases of the general problem, but most of them are intended for binary microstructures, and nearly all simulate only two-dimensional microstructures. The toolbox presented here attempts to exceed those limitations.
Its framework is a Metropolis stochastic-optimization routine running a simulated-anneal schedule, with particle position coordinates defining the configuration space and a range of forms available for the ﾓenergyﾔ? function. The toolbox allows several parameterizations of the microstructure, supplying all elementary properties (phase volume fractions, mean sizes, etc.) and some non-elementary properties (distributions of elementary properties, properties relating to inter-phase distances and morphology) of microstructures as possible parameters.
The toolbox is able, as one special case, to simulate realistic microstructures of uniaxially compacted mixtures of elemental Al-Ti-B powders and achieve basic microstructure-processing correlation. Statistical tests involving microstructural correlation functions bear out the realism. The toolbox is also able to generate virtual microstructures for the same system, for use in the design of experiments (which are in fact high-strain-rate impact simulations), and for evaluating hypotheses involving achievable material properties.
The Al-Ti-B powder compacts are potential advanced energetic materials that, when subjected to high-strain-rate impact (which may or may not constitute shock compression), explosively release heat by anaerobic reaction according as certain incompletely understood conditions are met or not. The study of those conditions and the mechanism of reaction initiation (carried out by a collaborator) is the specific application that the simulations in this work cater to.
To ensure realistic morphology in simulated Al-Ti-B microstructures, this work included reconstruction (carried out by montage serial sectioning) of large three-dimensional volumes of Al-Ti and Al-B binary compacts for two sets of powders that yielded actual 3 D Ti and B particle images. Accordingly, advancement of the experimental technique of montage serial sectioning and a quantitative characterization of the real powder microstructures also formed part of this research.
While only examples from Al-Ti-B powders are used throughout this work, it is clear that the…
Advisors/Committee Members: Gokhale, Arun M. (advisor), Frost, J. David (committee member), Thadhani, Naresh N. (committee member), Jang, Seung S. (committee member), Jacob, Karl I. (committee member).

► Auxetic materials are a rare class of materials that exhibit negative Poisson’s ratio. While most substances (like a rubber band) become thinner in lateral direction…
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▼ Auxetic materials are a rare class of materials that exhibit negative Poisson’s ratio. While most substances (like a rubber band) become thinner in lateral direction when stretched, auxetic materials grow thicker. The broad objective of this research is to study the origins of auxetic behavior in fibrous networks and to develop predictive processing-structure-property relations for these materials systems. We start by examining out-of-plane Poisson's ratio in paper by investigating a range of carefully chosen commercial paper samples. Laboratory handsheets were also produced and examined for their out-of-plane auxetic response. Geometrical and finite element models were built to help understand the origin of and underlying mechanism responsible for this auxetic response. Additionally, we were able to create a similar auxetic response in needle-punched nonwoven fiber networks by a heat-compression treatment. A series of microscopic and tomographic characterization was performed. From results on paper and nonwovens, it is evident that the type of network stabilization (hydrogen bonding in paper and needle-punching in nonwovens) and the choice of subsequent processing conditions have a significant influence on the out-of-plane Poisson’s ratio in these materials. Ultimately, a fundamental understanding of the origins of deformation behavior in these fiber networks should lead to the prospect of rational design of new auxetics and, in turn, to new product development opportunities for fiber-network materials.
Advisors/Committee Members: Griffin, Anselm C (advisor), Shofner, Meisha L (advisor), Bucknall, David G (committee member), Singh, Preet (committee member), Thadhani, Naresh (committee member), Wilkinson, Angus P (committee member).

► The temperature rise at the interface of two sliding bodies has significant bearing on the friction and wear characteristics of the bodies. The friction heat…
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▼ The temperature rise at the interface of two sliding bodies has significant bearing on the friction and wear characteristics of the bodies. The friction heat generated at the interface can be viewed as "loss of exergy" of the system, which also leads to accelerated wear in the form of oxidation, corrosion, and scuffing. This has a direct impact on the performance of the components or the machinery. If the sliding interface is also conducting electric current then the physics at the interface becomes complicated. The presence of electrical current leads to Joule heat generation at the interface along with other effects like electromotive, electroplasticity, stress relaxation and creep.
The interface of an electrical contact, either stationary or dynamic, is a complex environment as several different physical phenomena can occur simultaneously at different scales of observations. The main motivation for this work stems from the need to provide means for accurate determination or prediction of the critical contact parameters viz., temperature and contact resistance. Understanding the behavior of electrical contacts both static and dynamic under various operating conditions can provide new insights into the behavior of the interface. This dissertation covers three major topics: (1) temperature rise at the interface of sliding bodies, (2) study on static electrical contacts, and (3) study of factors influencing behavior of sliding electrical contacts under high current densities.
A model for determining the steady-state temperature distribution at the interface of two sliding bodies, with arbitrary initial temperatures and subjected to Coulomb and/or Joule heating, is developed. The model applies the technique of least squares regression to apply the condition of temperature continuity at every point in the domain. The results of the analysis are presented as a function of non-dimensional parameters of Peclet number, thermal conductivity ratio and ellipticity ratio. This model is first of its kind and enables the prediction of full temperature field. The analysis can be applied to a macro-scale contact, ignoring surface roughness, between two bodies and also to contact between two asperities. This analysis also yields an analytical expression for determining the heat partition between two bodies, if the Jaeger's hypothesis of equating average temperatures of both the bodies is being implemented.
In general for design purposes one is interested in either the maximum or the average temperature rise at the interface of two sliding bodies. Jaeger had presented simple equations, based on matching the average temperatures of both bodies, for square and band shaped contact geometries. Engineers since then have been using those equations for determining the interface temperature for circular and elliptical shaped contact geometries. Curve fit equations for determining the maximum and the average interface temperature for circular and elliptical contact with semi-ellipsoidal form of heat distribution are presented. These…
Advisors/Committee Members: Streator, Jeffrey (Committee Chair), Blanchet, Thierry (Committee Member), Cowan, Richard (Committee Member), Danyluk, Steven (Committee Member), Neu, Richard (Committee Member), Thadhani, Naresh (Committee Member).

► In this dissertation, the role of disorder in determining the physical properties of materials in the AM2O8 and AM2O7 families was investigated. A link was…
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▼ In this dissertation, the role of disorder in determining the physical properties of materials in the AM2O8 and AM2O7 families was investigated. A link was established between orientational disorder and the thermoelastic properties of these materials. It was also demonstrated that phase transition temperatures in these materials are noticeably pressure-dependent. In Chapters 3 and 4, it was shown that compression-induced orientational disorder was correlated with temperature-dependent bulk moduli and pressure-dependent CTEs in the orientationally ordered phases of ZrW2O8 and HfW2O8 . No changes in orientational order were observed in ZrMo2O8 or the orientationally disordered phases of ZrW2O8 and HfW2O8 upon compression, and the bulk moduli and CTEs were relatively temperature- and pressure-independent respectively. The pressure-sensitivity of the CTE has implications for the use of these materials in controlled thermal expansion composites, since internal stresses on par with the pressure range examined can be induced by mismatches in thermal expansion between components of the composite. In Chapter 5, it was demonstrated that the CTE of the orientationally ordered rhombohedral phase of SnMo2O8 was pressure-sensitive, while that of the orientationally disordered cubic phase was not. Additionally, at temperatures near the ambient pressure rhombohedral → cubic transition, it was possible to interconvert between these two phases upon compression and decompression. The phase transition pressure was also found to be elevated significantly by slight increases in temperature. Both phases were significantly softer than all phases of ZrW2O8, ZrMo2O8, and HfW2O8. In Chapter 6, it was demonstrated both the supercell → incommensurate and incommensurate → subcell transition temperatures of ZrV2O7 and HfV2O7 are extremely pressure-sensitive, to the point where NTE was not observed below 513K at pressures above 155 MPa. Additionally, the CTEs of both the low temperature and high temperature phases were strongly-pressure-dependent. This is due in part to the close proximity of the supercell → incommensurate and incommensurate → subcell transitions, which are both associated with large volume changes. The high temperature phase was found to be much stiffer than the low temperature phase. In Chapter 7, it was demonstrated that ZrAs2O7 and HfAs2O7 exhibit PTE between 100K and 500K. Their thermal expansion behavior was found to be more similar to that of ZrP2O7 than that of ZrV2O7. The crystal structure of ZrAs2O7and HfAs2O7 could not be determined from high resolution XRD data; however, these phases were determined to be of lower symmetry than previously reported.
Advisors/Committee Members: Wilkinson, Angus P. (advisor), Perry, Joseph W. (committee member), El-Sayed, Mostafa A. (committee member), Zhang, Z. John (committee member), Thadhani, Naresh N. (committee member).

► Novel intracellular drug delivery techniques are needed to overcome the barrier of the cell’s plasma membrane. In this study, we leveraged a novel, laser-mediated technique…
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▼ Novel intracellular drug delivery techniques are needed to overcome the barrier of the cell’s plasma membrane. In this study, we leveraged a novel, laser-mediated technique known as transient nanoparticle energy transduction (TNET), in which carbon black (CB) nanoparticles in suspension with DU145 cells and small molecules irradiated by nanosecond-pulsed near infrared (NIR) laser energy leads to efficacious delivery and high cell viability. To gain mechanistic insight into TNET, we studied various aspects of this in vitro system, including cellular mechanics, total energy input, and the role of photoacoustics. First, we studied the role of cellular mechanics in TNET by way of the cytoskeleton and plasma membrane fluidity. From these studies, we concluded that cytoskeletal mechanics are integral to resulting bioeffects achieved with TNET, whereas the fluidity of the plasma membrane is not. Next, we studied the effect of energy input into the system, which was increased by increasing laser fluence, CB nanoparticle concentration and number of laser pulses. We found that total energy input strongly correlated with resulting bioeffects. Lastly, we studied the effects of three different carbon-based nanoparticles – CB, multi-walled carbon nanotubes (MWCNT) and single-walled (SWCNT) carbon nanotubes – on cellular bioeffects. In addition to the different bioeffect profiles, CB, MWCNT, and SWCNT also exhibited differences in the intensity of photoacoustic output in the form of a single, mostly positive-pressure pulse of ~100 ns duration. Lack of a universal correlation between peak pressure and cellular bioeffects, suggested that total energy input rather than pressure output was more mechanistically relevant to TNET. Overall, this work provides functional characterization and mechanistic understanding the cellular bioeffects cause by TNET. These studies will contribute to a necessary understanding of TNET that will enable rational design of TNET systems for future applications and possible translation into the clinic.
Advisors/Committee Members: Thadhani, Naresh N. (advisor), Prausnitz, Mark R. (advisor), Milam, Valeria (committee member), Bucknall, David (committee member), LaPlaca, Michelle (committee member), Gray, Michael D. (committee member).

► The effect of viscoplastic deformation of the energetic material HMX on the mechanical, thermal, and ignition response of a two-phase (HMX and Estane) polymer bonded…
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▼ The effect of viscoplastic deformation of the energetic material HMX on the mechanical, thermal, and ignition response of a two-phase (HMX and Estane) polymer bonded explosive (PBX) is analyzed. Specific attention is given to the high strain rate response of the material during the first passage of the stress wave when impacted by a constant velocity piston. PBX microstructures are subjected to impact loading from a constant velocity piston traveling at a rate of 50 to 200 m/s using a 2D cohesive finite element (CFEM) framework. The initial focus is to fully quantify the effect that viscoplastic HMX has on the behavior of a PBX composite, a thorough thermo-mechanical analysis is performed. The thermal response of the PBX specimens having viscoplastic HMX is characterized by a significant reduction in average heating, peak temperature rise, and the number or amount of material experiencing localized heating (hotspots). This reduction in heating is found to be accomplished through the mechanism of greatly reducing the density of fracture in the PBX. The second focus of this work is to evaluate the ignition sensitivity of these materials to determine the effect, if any, of the viscoplastic HMX. Viscoplastic HMX is shown to increase the minimum load duration, mean load duration, and range of critical load durations required for ignition. A 3D crystal plasticity framework is employed to quantify the potential heterogeneities in the stress and temperature field resulting from the inherent crystalline anisotropy of the HMX grains. It is found that in a densely packed HMX, the heterogeneities due to material anisotropy can contribute to increased stress gradients and localized temperature rise. Finally, the 2D framework is used to study a hypothetical composite containing HMX grains suspended in an aluminum matrix. This investigation focuses not on the feasibility of producing such a composite, but on determining whether such an arrangement would be advantageous from a mechanical and ignition sensitivity standpoint. Results indicate that this hypothetical composite would be considerably less sensitive than a similar PBX.
Advisors/Committee Members: Zhou, Min (advisor), Neu, Richard (committee member), Lindsay, Michael (committee member), Thadhani, Naresh (committee member), Rimoli, Julian J. (committee member).

Hardin, D. B. (2015). The role of viscoplasticity in the deformation and ignition response of polymer bonded explosives. (Doctoral Dissertation). Georgia Tech. Retrieved from http://hdl.handle.net/1853/53516

Hardin, David Barrett. “The role of viscoplasticity in the deformation and ignition response of polymer bonded explosives.” 2015. Web. 25 May 2019.

Vancouver:

Hardin DB. The role of viscoplasticity in the deformation and ignition response of polymer bonded explosives. [Internet] [Doctoral dissertation]. Georgia Tech; 2015. [cited 2019 May 25].
Available from: http://hdl.handle.net/1853/53516.

Council of Science Editors:

Hardin DB. The role of viscoplasticity in the deformation and ignition response of polymer bonded explosives. [Doctoral Dissertation]. Georgia Tech; 2015. Available from: http://hdl.handle.net/1853/53516

► In this research, we study two different geometric approaches, namely, the discrete exterior calculus and differential complexes, for developing numerical schemes for linear and nonlinear…
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▼ In this research, we study two different geometric approaches, namely, the discrete exterior calculus and differential complexes, for developing numerical schemes for linear and nonlinear elasticity. Using some ideas from discrete exterior calculus (DEC), we present a geometric discretization scheme for incompressible linearized
elasticity. After characterizing the configuration manifold of volume- preserving discrete deformations, we use Hamilton’s principle on this configuration manifold. The discrete Euler-Lagrange equations are obtained without using Lagrange multipliers.
The main difference between our approach and the mixed finite element formulations is that we simultaneously use three different discrete spaces for the displacement field. We test the efficiency and robustness of this geometric scheme using some numerical examples. In particular, we do not see any volume locking and/or checkerboarding of pressure in our numerical examples. This suggests that our choice of discrete solution
spaces is compatible. On the other hand, it has been observed that the linear elastostatics complex can be used to find very efficient numerical schemes. We use some geometric techniques to obtain differential complexes for nonlinear elastostatics.
In particular, by introducing stress functions for the Cauchy and the second Piola-Kirchhoff stress tensors, we show that 2D and 3D nonlinear elastostatics admit separate kinematic and kinetic complexes. We show that stress functions corresponding to the first Piola-Kirchhoff stress tensor allow us to write a complex for 3D nonlinear
elastostatics that similar to the complex of 3D linear elastostatics contains both the kinematics an kinetics of motion. We study linear and nonlinear compatibility equations
for curved ambient spaces and motions of surfaces in R3. We also study the relationship between the linear elastostatics complex and the de Rham complex. The geometric approach presented in this research is crucial for understanding connections
between linear and nonlinear elastostatics and the Hodge Laplacian, which can enable one to convert numerical schemes of the Hodge Laplacian to those for linear and possibly nonlinear elastostatics.
Advisors/Committee Members: Yavari, Arash (advisor), Desroches, Reginald (committee member), Gangbo, Wilfrid (committee member), Garmestani, Hamid (committee member), Thadhani, Naresh (committee member).

▼ Energetic structural materials (ESMs) constitute a new class of materials that provide dual functions of strength and energetic characteristics. ESMs are typically composed of micron-scale or nano-scale intermetallic mixtures or mixtures of metals and metal oxides, polymer binders, and structural reinforcements. Voids are included to produce a composite with favorable chemical reaction characteristics.
In this thesis, a continuum approach is used to simulate gas-gun or explosive loading experiments where a strong shock is induced in the ESM by an impacting plate. Algorithms are developed to obtain equations of state of mixtures. It is usually assumed that the shock loading increases the energy of the ESM and causes the ESM to reach the transition state. It is also assumed that the activation energy needed to reach the transition state is a function of the temperature of the mixture. In this thesis, it is proposed that the activation energy is a function of temperature and the stress state of the mixture. The incorporation of such an activation energy is selected in this thesis. Then, a multi-scale chemical reaction model for a heterogeneous mixture is introduced. This model incorporates reaction initiation, propagation, and extent of completed reaction in spatially heterogeneous distributions of reactants. A new model is proposed for the pore collapse of mixtures. This model is formulated by modifying the Carol, Holt, and Nesterenko spherically symmetric model to include mixtures and compressibility effects.
Uncertainties in the model result from assumptions in formulating the models for continuum relationships and chemical reactions in mixtures that are distributed heterogeneously in space and in numerical integration of the resulting equations. It is important to quantify these uncertainties. In this thesis, such an uncertainty quantification is investigated by systematically identifying the physical processes that occur during shock compression of ESMs which are then used to construct a hierarchical framework for uncertainty quantification.
Advisors/Committee Members: Hanagud, Sathya (Committee Chair), Kardomateas, George (Committee Member), McDowell, David (Committee Member), Ruzzene, Massimo (Committee Member), Thadhani, Naresh (Committee Member).

► Mo-Si-B alloys are a leading candidate for the next generation of jet turbine engine blades and have the potential to raise operating temperatures by 300-400°C.…
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▼ Mo-Si-B alloys are a leading candidate for the next generation of jet turbine engine blades and have the potential to raise operating temperatures by 300-400°C. The alloys of interest are a three-phase mixture of the molybdenum solid solution (Moss) and two intermetallic phases, Mo3Si (A15) and Mo5SiB2 (T2). A novel powder metallurgical method was developed which uses the reaction of molybdenum, silicon nitride (Si3N4) and boron nitride (BN) powders to synthesize a fine dispersion of intermetallics in a Moss matrix. The covalent nitrides are stable in oxidizing environments up to 1000ºC, allowing for fine particle processing. The process developed uses standard powder processing techniques to create Mo-Si-B alloys in a less complex and expensive manner than previously demonstrated.
This powder metallurgy approach yields a fine dispersion of intermetallics in the Moss matrix with average grain sizes of 2-4μm. Densities up to 95% of theoretical were attained from pressureless sintering at 1600°C and full theoretical density was achieved by hot-isostatic pressing (HIP). Sintering and HIPing at 1300°C reduced the grain sizes of all three phases by over a factor of two.
Microstructure examination by electron back-scatter diffraction imaging was used to precisely define the location of the phases and to measure the volume fractions and grain size distributions. Microstructural quantification techniques including two-point correlation functions were used to quantify microstructural features and correlate the BN reactant powder size and morphology to the distribution of the intermetallic phases.
High-temperature tensile tests were conducted and yield strengths of 580MPa at 1100°C and 480MPa at 1200°C were measured for the Mo-2Si-1Bwt.% alloy. The yield strength of the Mo-3Si-1Bwt.% alloy was 680MPa at 1100°C and 420MPa at 1300°C. A review of the pertinent literature reveals that these are among the highest yield strengths measured for these compositions.
The oxidation resistance in air at 1000 and 1100°C was examined. The protective borosilicate surface layer formed quickly due to the close spacing of intermetallic particles and pre-oxidation treatment was developed to further limit the transient oxidation behavior. An oxidation model was developed which factors in the different stages of oxidation to predict compositions that minimize oxidation.
Advisors/Committee Members: Cochran, Joe (Committee Chair), Berczik, Doug (Committee Member), Sanders, Tom (Committee Member), Sandhage, Ken (Committee Member), Thadhani, Naresh (Committee Member).

► High explosive charges when detonated ensue in a flow field characterized by several physical phenomena that include blast wave propagation, hydrodynamic instabilities, real gas effects,…
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▼ High explosive charges when detonated ensue in a flow field characterized by several physical phenomena that include blast wave propagation, hydrodynamic instabilities, real gas effects, fluid mixing and afterburn effects. Solid metal particles are often added to explosives to augment the total impulsive loading, either through direct bombardment if inert, or through afterburn energy release if reactive. These multiphase explosive charges, termed as heterogeneous explosives, are of interest from a scientific perspective as they involve the confluence and interplay of various additional physical phenomena such as shock-particle interaction, particle dispersion, ignition, and inter-phase mass, momentum and energy transfer.
In the current research effort, chemical explosions in multiphase environments are investigated using a robust, state-of-the-art Eulerian-gas, Lagrangian-solid methodology that can handle both the dense and dilute particle regimes. Explosions into ambient air as well as into aluminum particle clouds are investigated, and hydrodynamic instabilities such as Rayleigh- Taylor and Richtmyer-Meshkov result in a mixing layer where the detonation products mix with the air and afterburn. The particles in the ambient cloud, when present, are observed to pick up significant amounts of momentum and heat from the gas, and thereafter disperse, ignite and burn. The amount of mixing and afterburn are observed to be independent of particle size, but dependent on the particle mass loading and cloud dimensions. Due to fast response times, small particles are observed to cluster as they interact with the vortex rings in the mixing layer, which leads to their preferential ignition/ combustion.
The total deliverable impulsive loading from heterogeneous explosive charges containing inert steel particles is estimated for a suite of operating parameters and compared, and it is demonstrated that heterogeneous explosive charges deliver a higher near-field impulse than homogeneous explosive charges containing the same mass of the high explosive. Furthermore, particles are observed to introduce significant amounts of hydrodynamic instabilities in the mixing layer, resulting in augmented fluctuation intensities and fireball size, and different growth rates for heterogeneous explosions compared to homogeneous explosions. For aluminized explosions, the particles are observed to burn in two regimes, and the average particle velocities at late times are observed to be independent of the initial solid volume fraction in the explosive charge. Overall, this thesis provides useful insights on the role played by solid particles in chemical explosions.
Advisors/Committee Members: Menon, Suresh (Committee Chair), Jagoda, Jeff (Committee Member), Ruffin, Stephen (Committee Member), Thadhani, Naresh (Committee Member), Walker, Mitchell (Committee Member).

Balakrishnan, K. (2010). On the high fidelity simulation of chemical explosions and their interaction with solid particle clouds. (Doctoral Dissertation). Georgia Tech. Retrieved from http://hdl.handle.net/1853/34672

Balakrishnan, Kaushik. “On the high fidelity simulation of chemical explosions and their interaction with solid particle clouds.” 2010. Web. 25 May 2019.

Vancouver:

Balakrishnan K. On the high fidelity simulation of chemical explosions and their interaction with solid particle clouds. [Internet] [Doctoral dissertation]. Georgia Tech; 2010. [cited 2019 May 25].
Available from: http://hdl.handle.net/1853/34672.

Council of Science Editors:

Balakrishnan K. On the high fidelity simulation of chemical explosions and their interaction with solid particle clouds. [Doctoral Dissertation]. Georgia Tech; 2010. Available from: http://hdl.handle.net/1853/34672

► In this thesis, we pursue a simulation-based approach whereby microstructure-sensitive finite element simulations are performed within a statistical perspective to examine the VHCF life variability…
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▼ In this thesis, we pursue a simulation-based approach whereby microstructure-sensitive finite element simulations are performed within a statistical perspective to examine the VHCF life variability and assess the surface initiation probability. The methodology introduced in this thesis lends itself as a cost-effective platform for development of microstructure-property relations to support design of new or modified alloys, or to more accurately predict the properties of existing alloys.
Advisors/Committee Members: McDowell, David (Committee Chair), Garmestani, Hamid (Committee Member), Larsen, James (Committee Member), Neu, Richard (Committee Member), Thadhani, Naresh (Committee Member).

► Shock compaction experiments were performed on soft magnetic phases Fe₄N and Fe₁₆N₂, and hard magnetic phases Nd₂Fe₁₄B and Sm₂Fe₁₇N₃ in order to determine their thermo-mechanical…
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▼ Shock compaction experiments were performed on soft magnetic phases Fe₄N and Fe₁₆N₂, and hard magnetic phases Nd₂Fe₁₄B and Sm₂Fe₁₇N₃ in order to determine their thermo-mechanical stability during shock loading and explore the possibility of fabricating a textured nanocomposite magnet. Gas gun experiments performed on powders pressed in a three capsule fixture showed phase transformations occurring in Fe₄N, Fe₁₆N₂, and Nd₂Fe₁₄B, while Sm₂Fe₁₇N₃ was observed to be relatively stable. Shock compaction of FCC Fe₄N resulted in a partial transformation to HCP Fe₃N, consistent with previous reports of the transition occurring at a static pressure of ~3 GPa. Shock compaction of Fe₁₆N₂ produced decomposition products alpha-Fe, Fe₄N, and FeN due to a combination of thermal effects associated with dynamic void collapse and plastic deformation. Decomposition of Nd-Fe-B, producing alpha-Fe and amorphous Nd-Fe-B, was observed in several shock consolidated samples and is attributed to deformation associated with shock compaction, similar to decomposition reported in ball milled Nd-Fe-B. No decomposition was observed in shock compacted samples of Sm-Fe-N, which is consistent with literature reports showing decomposition occurring only in samples compacted at a pressure above ~15 GPa. Nd-Fe-B and Sm-Fe-N were shown to accommodate deformation primarily by grain size reduction, especially in large grained materials. Hard/Soft composite magnetic materials were formed by mixing single crystal particles of Nd-Fe-B with iron nanoparticles, and the alignment-by-magnetic-field technique was able to introduce significant texture into green compacts of this mixture. While problems with decomposition of the Nd₂Fe₁₄B phase prevented fabricating bulk magnets from the aligned green compacts, retention of the nanoscale morphology of the alpha-Fe particles and the high alignment of the green compacts shows promise for future development of textured nanocomposite magnets through shock compaction.
Advisors/Committee Members: Thadhani, Naresh (Committee Chair), Garmestani, Hamid (Committee Member), Matthew Willard (Committee Member), Sanders, Thomas (Committee Member), Suryanaray Sankar (Committee Member).

► The effects of reactive metal particles on the microstructure and mechanical properties of epoxy-based composites are investigated in this work. To examine these effects castings…
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▼ The effects of reactive metal particles on the microstructure and mechanical properties of epoxy-based composites are investigated in this work. To examine these effects castings of epoxy reinforced with 20-40 vol.% Al and 0-10 vol.% Ni were prepared, while varying the aluminum particle size from 5 to 50 microns and holding the nickel particle size constant at 50 microns. In total eight composite materials were produced, possessing unique microstructures. The microstructure is quantitatively characterized and correlated with the composite constitutive response determined from quasi-static and dynamic compressive loading conditions at strain-rates from 1e-4 to 5e3 /s. Microstructures from each composite and at each strain rate were analyzed to determine the amount of particle strain as a function of bulk strain and strain rate. Using computational simulations of representative microstructures of select composites, the epoxy matrix-metallic particle and particle-particle interactions at the mesoscale under dynamic compressive loading conditions were further examined. From computational simulation data, the stress and strain localization effects were characterized at the mesoscale and the bulk mechanical behavior was decomposed into the individual contributions of the constituent phases. The particle strain and computational analysis provided a greater understanding of the mechanisms associated with particle deformation and stress transfer between phases, and their influence on the overall mechanical response of polymer matrix composites reinforced with metallic particles. The highly heterogeneous composite microstructure and the high contrasting properties of the individual constituents were found to drive localized deformations that are often more pronounced than those in the bulk material. The strain rate behavior of epoxy is shown to cause a strain rate dependent deformation response of reinforcement particle phases that are typically strain rate independent. Additionally, the epoxy matrix strength behavior was found to have a higher dependence on strain rate due to the presence of metal particle fillers. Discrepancies between experimental and simulation mechanical behavior results and these findings indicate a need for epoxy constitutive models to incorporate effects of particle reinforcement on the mechanical behavior.
Advisors/Committee Members: Thadhani, Naresh (Committee Chair), Gokhale, Arun (Committee Member), Jordan, Jennifer (Committee Member), Spowart, Jonathan (Committee Member), Tsukruk, Vladimir (Committee Member).

► The mechanical behavior of structural materials used in nuclear applications is significantly degraded as a result of irradiation, typically characterized by an increase in yield…
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▼ The mechanical behavior of structural materials used in nuclear applications is significantly degraded as a result of irradiation, typically characterized by an increase in yield stress, localization of inelastic deformation along narrow dislocation channels, and considerably reduced strains to failure. Further, creep rates are accelerated under irradiation. These changes in mechanical properties can be traced back to the irradiated microstructure which shows the formation of a large number of material defects, e.g., point defect clusters, dislocation loops, and complex dislocation networks. Interaction of dislocations with the irradiation-induced defects governs the mechanical behavior of irradiated metals. However, the mechanical properties are seldom systematically correlated to the underlying irradiated microstructure. Further, the current state of modeling of deformation behavior is mostly phenomenological and typically does not incorporate the effects of microstructure or defect densities.
The present research develops a continuum constitutive crystal plasticity framework to model the mechanical behavior and deformed microstructure of bcc ferritic/martensitic steels exposed to irradiation. Physically-based constitutive models for various plasticity-induced dislocation migration processes such as climb and cross-slip are developed. We have also developed models for the interaction of dislocations with the irradiation-induced defects. A rate theory based approach is used to model the evolution of point defects generated due to irradiation, and coupled to the mechanical behavior. A void nucleation and growth based damage framework is also developed to model failure initiation in these irradiated materials. The framework is used to simulate the following major features of inelastic deformation in bcc ferritic/martensitic steels: irradiation hardening, flow localization due to dislocation channel formation, failure initiation at the interfaces of these dislocation channels and grain boundaries, irradiation creep deformation, and temperature-dependent non-Schmid yield behavior. Model results are compared to available experimental data.
This framework represents the state-of-the-art in constitutive modeling of the deformation behavior of irradiated materials.
Advisors/Committee Members: McDowell, David L. (advisor), Deo, Chaitanya (committee member), Garmestani, Hamid (committee member), Thadhani, Naresh (committee member), Zhu, Ting (committee member).